1. Field of the Invention
The present invention relates to a honeycomb unit support structure for an electrically-heated catalyst apparatus, and more in particular to a metal honeycomb unit support structure constituting a catalyst carrier of an electrically-heated catalyst apparatus which can be supplied with power and heated to a catalyst activation temperature within a short time.
2. Description of the Related Art
The exhaust gas emitted from the internal combustion engine mounted on an automotive vehicle contains hazardous materials such as HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxides). The exhaust manifold of the internal combustion engine, therefore, is generally provided with a catalyst converter as an exhaust gas cleaning unit for cleaning the exhaust gas. The three-way catalyst used for the catalyst converter, however, is known to have a low rate of cleaning hazardous materials in the exhaust gas when the catalyst temperature is low (is in an inactive state). The exhaust gas cannot be cleaned sufficiently, therefore, as long as the catalyst is inactive after cold starting of the internal combustion engine.
In view of this, an electrically-heated catalyst apparatus has been suggested in which a catalyst carrier made of a metal is supplied with electrical power at the time of starting the engine or when the catalyst is inactive, so that the metal catalyst carrier is heated up to a catalyst activation temperature (300.degree. to 400.degree. C.) within a short time.
The electrically-heated catalyst apparatus comprises a cylindrical casing connected to the exhaust manifold of the internal combustion engine, and a main catalyst carrier and an electrically-heated catalyst carrier serially arranged in spaced relation to each other in the cylindrical casing. The electrically-heated catalyst carrier is heated by power supplied thereto and includes a central electrode which is led out of the casing through an electrode section. A battery and a switch are connected between the electrode section and an external electrode provided on a part of the casing. When the switch is turned on, the electrically-heated catalyst carrier is supplied with power thereby to generate heat.
The electrically-heated catalyst carrier is generally made by superposing a corrugated metal foil (hereinafter called "the corrugated foil") and a tabular metal foil (hereinafter called "the tabular foil") on each other, coupling the longitudinal ends thereof to the center electrode, and then winding the superposed structure of the corrugated foil and the tabular foil around the central electrode thereby to constitute a spiral metal foil laminate (honeycomb unit). A foil member about 50 .mu.m thick made of an iron alloy containing aluminum is generally used for the corrugated foil or the tabular foil.
The honeycomb unit 7 made by winding the corrugated foil and the tabular foil around the central electrode has an axial path formed spirally around the central electrode by the space between the corrugated foil and the tabular foil. Also, an exhaust gas cleaning catalyst is carried on the surface of the corrugated foil and the tabular foil. When the switch is turned on for supplying power to the honeycomb unit and the exhaust gas is caused to flow through the path while the temperature of the catalyst is increased for activation, the hazardous components of the exhaust gas are brought into contact with the catalyst and the exhaust gas is cleaned.
The electrically-heated catalyst apparatus configured in this way has the disadvantage that the corrugated foil and the tabular foil of the honeycomb unit are liable to be displaced from each other (hereinafter referred to as "telescoping") or are liable to break down due to such adverse factors as the high-temperature, high-speed exhaust gas flowing in the exhaust manifold, the heat cycle in which the engine is heated when running and cooled when stationary, and vibrations exerted from the engine or the vehicle body, thereby often leading to a reduced function of the electrically-heated catalyst carrier.
The following solutions to these problems have been proposed:
(1) The end surface of the honeycomb unit of the electrically-heated catalyst apparatus downstream of the exhaust gas flow is supported in contact with at least a support bar (retainer) covered with an insulating material, and the retainer is welded to a metal outside cylinder (case) (see JP-A-5-269389).
(2) A joint reinforcement is formed on each of the inner periphery and the outer periphery of the honeycomb unit of the electrically-heated catalyst apparatus, an insulative ceramics bar is suspended as a retainer over the inner peripheral reinforcement and the outer peripheral reinforcement downstream of the exhaust gas flow for preventing the catalyst carrier moving backward, and each retainer is fixed by a support member thereby to prevent telescoping (foil displacement) of the electrically-heated catalyst carrier.
(3) The contact portion between the corrugated foil and the tabular foil in the neighborhood of each of the outer and inner peripheries of the honeycomb unit is coupled by welding with a Ni solder or the like, and a reinforcement thus is provided on each of the outer and inner peripheries of the honeycomb unit.
The preventive means first-above described, however, poses the problem that the honeycomb unit portion free of the retainer suffers telescoping. The second preventive means, on the other hand, if the support members are mounted inappropriately, develops an uneven temperature distribution on the outer cylinder and deforms the support members, often causing the retainers to come off from the support members.
Further, the problem of the third preventive means is that telescoping occurs in the boundary between the reinforced outer peripheral portion and the remaining portions of the honeycomb unit. Telescoping also occurs in similar fashion in the neighborhood of the boundary between the honeycomb unit having no inner or outer peripheral reinforcement and the case containing the honeycomb unit.